This invention relates to a method and apparatus for determining the recording time available on a videotape cassette; and, in particular, to an apparatus for determining the remaining recording time on a partially recorded videotape cassette.
Conventional videotape cassettes, such as VHS format cassettes available commercially for time-displacement recording of television programs in the home, comprise rectangular casings within which are protectively housed fixed lengths of magnetic video recording tape mounted for transportation between supply and take-up reels, across a video cassette recorder (VCR) read-write recording head. Such tapes come in T30, T60, T120, T130 and T160 different tape sizes, respectively corresponding to different 30, 60, 120, 130 and 160 minutes of nominally available recording time. The actual recording time available on a particular tape is a multiple of the nominal recording time determined by the recording speed (SP, LP or EP).
There is a problem when you want to record several long programs, such as full-length movies, on a single tape. If the tape is already partially recorded, there may not be enough recording time left to get the whole program. So, either the end will be missed, or the tape will rewind automatically and begin recording over the first program, depending on the type of VCR you use. One way to guard against this is to use a new tape each time you record; but, this wastes tape. Another way is to set the counter on the VCR and keep track of used and unused tape that way. That is cumbersome and it is easy to forget to reset the tape counter. A more convenient way is to be able to determine how much recording time is left, just by measuring the amount of unrecorded tape left on the supply reel, or amount of recorded tape taken up on the take-up reel.
Existing ways for measuring the amount of tape on the supply or take-up reel are unsatisfactory. The cassette housing itself has see-through viewing ports windows above the reels with markings at radially-spaced increments, but these are hard to see and do not include time identifications. Moreover, because the reels are merely captured and not fixed against translational movement within the cassette housing, considerable float exists between the reel hubs and the markings, making such readings inaccurate. Stickers, such as shown in U.S. Pat. No. 4,501,396, can be applied to the windows to improve readability; however, they do not accommodate the float. Rulers, such as shown in U.S. Pat. No. 4,893,414, exist that can be placed over a window and lined up with the hub. Such rulers have markings identified by increments of unrecorded/recorded time associated with corresponding different radii of tape loaded on the hub. The linear scales on such direct reading devices are, however, too crowded to be useful in a close case.